According to advance of a liquid crystal panel manufacturing technique, liquid crystal display elements are widely used in field of an optical information process.
Conventionally, a Twisted Nematic (TN) display method is most frequently applied as a liquid crystal display element applied to small and medium displays and this is a technique of installing electrodes on two substrates, respectively, arranging liquid crystal directors to be twisted by 90°, and then applying voltage to the electrodes to operate the liquid crystal directors.
The TN liquid crystal display element provides excellent contrast and color reproducibility, and a vertical alignment (VA) mode liquid crystal display element in which longitudinal axes of liquid crystal molecules are arranged to be vertical to upper and lower display plates in a state where electric field is not applied has a high contrast ratio and has been prominent. However, the TN liquid crystal display element has a problem of a narrow viewing angle.
In order to solve such a problem of a viewing angle in the TN manner, a patterned vertically aligned mode (PVA mode) of applying an incision to a vertical alignment mode liquid crystal display device, and an in-plane switching mode (IPS mode) of forming two electrodes on one substrate and adjusting directors of liquid crystal by transverse electric field generated between two electrodes have been introduced.
Then, in order to improve a low opening ratio and transmissivity of the IPS mode, a fringe field switching mode (FFS mode) of forming a gap between a counter electrode and a pixel electrode to be narrow while forming the counter electrode and the pixel electrode are formed of transparent conductivities, and operating liquid crystal molecules by fringe field formed between the counter electrode and the pixel electrode has been emerged.
Meanwhile, in order to solve a problem that optical efficiency of the FFS mode is lower than that of the TN mode, an FIS mode was developed, thus it is possible to improve low transmissivity between pixel electrodes in the conventional FFS mode and to achieve a liquid crystal display element capable of operating with low voltage in an voltage applying manner through two thin film transistors.
In addition, each of these modes has unique liquid crystal arrangement and optical anisotropy. Accordingly, in order to compensate for phase difference based on the optical anisotropy of the liquid crystal modes, an optical phase difference film with optical anisotropy corresponding to each mode is required. The optical phase difference film was developed as a color compensation film of an LCD, but recently, more variety of functions such as high-wavelength ecentralization, wide viewing angle, temperature compensation, and high phase difference value film are required.
In the case of a liquid crystal display device, it is general that alignment of liquid crystal molecules controlled in advance is changed to another alignment state by applying electric field, a polarization direction or a polarization state of passing light is changed, and this change is displayed by changing to contrast of brightness by a polarizer or the like.
As a general method of aligning liquid crystal, a contact rubbing method of applying a polymer film such as polyimide to a substrate such as glass and rubbing this surface in a predetermined direction with fiber such as nylon polyester is used. Liquid alignment based on the contact rubbing method described above has an advantage of obtaining simple and stable alignment performance of liquid crystal. However, when the polymer film is rubbed with fiber, fine dust and electrostatic discharge (ESD) occurs to damage the substrate and, according to enlargement of rolls caused by increase in processing time and enlargement of glass, a significant problem may be caused during manufacturing a liquid crystal panel due to difficulty in process such as unevenness of rubbing strength.
In order to solve the problem of the contact rubbing method described above, new methods for manufacturing a non-contact alignment film have been actively studied. As the methods for manufacturing a non-contact alignment film, there are an optical alignment method, an energy beam alignment method, a vapor deposition alignment method, an etching method using lithography, and the like.
Particularly, the optical alignment method means mechanism of forming a photopolymerization liquid crystal alignment film in which a photoreactive material coupled to photoreactive polymer causes optical reaction by linearly polarized UV to have predetermined arrangement, thereby aligning liquid crystal finally.
To this end, when linearly polarized UV is irradiated, a photoreactive material has to have characteristics of being arranged at a predetermined angle and in a predetermined direction according to a polarization direction, and has to be well matched to reactive liquid crystal to well achieve liquid crystal alignment by interaction with reactive liquid crystal. Particularly, an optical alignment material forming an optical alignment film has to good in physical properties such as printing property, alignment stability, and thermal stability.
As optical reaction based on UV irradiation, photopolymerization reaction of cinnamate, coumarin, chalcone, stilbene, diazoacetic, and the like, photoisomerizable reaction of cis-trans isomerization, and molecular chain break of decomposition have been already known. There are examples of applying such molecular optical reaction based on UV to liquid crystal alignment based on UV irradiation through design of appropriate alignment film molecules and optimization of UV irradiation conditions.
For example, Korean Registered Patent Publication No. 10-0423213 discloses a manufacturing method of a liquid crystal alignment film and a liquid crystal display element having the liquid crystal alignment film, wherein alignment performance is granted by irradiation of linearly polarized UV without performing a rubbing process. Particularly, in Japan, Korea, Europe, USA, and the like related to LCD industries, a lot of patents related to such optical alignment methods have been applied. However, after deriving an initial idea, some have been in mass production, but would not have been generally and widely applied in industrial field.
This is because simple liquid crystal alignment can be induced by the optical reaction but stable alignment characteristics cannot be maintained or provided in terms of external heat, light, physical impact, chemical impact, and the like. In other words, the optical alignment method is low in productivity or reliability as compared with the rubbing method. The main causes of such problems are alignment anchoring energy lower than that of the rubbing method, low alignment stability of liquid crystal, and the like.